(1) Field of the Invention
The present invention relates generally to a vacuum circuit breaker, and more particularly to a vacuum power interrupting device for interrupting a large current flow having at least one bell-shaped vacuum power interrupter unit which comprises a vacuum vessel wherein an opening portion of a cup-shaped metallic casing is hermetically sealed with an insulating circular end plate made of a ceramic material and a pair of electrical contacts are installed in the axial direction of the vacuum power interrupter; one is a stationary electrical contact and the other is a movable electrical contact, so that normally they are in contact with each other but the latter is drawn away from the former during a current interruption.
(2) Description of the Prior Art
As compared with a conventional vacuum circuit breaker having at least one vacuum power interrupter unit of the type comprising a vacuum vessel hermetically sealed with a metallic end plate at each end of a cylindrical insulating envelope wherein stationary and movable electrical contacts are installed so that they are normally in contact with each other, but the latter can be moved away from the former, the bell-shaped vacuum interrupter unit described above can open or close a larger current with a high voltage simply by increasing the diameter of the vacuum vessel, that is, the opening end of the bell-shaped metallic casing and the diameter portion of insulating circular end plate fitted thereinto. Simultaneously, the bell-shaped vacuum power interrupter unit can be less expensive and can be fabricated easily by replacing the expensive insulating envelope used in the conventional type described above with such an inexpensive insulating circular end plate made of a single ceramic material.
However, such a bell-shaped vacuum power interrupter unit constituting the circuit breaker has the problem that the outer surface of the insulating circular end plate partially surrounding the vacuum vessel corresponds to an atmospheric creepage distance from an electrically charged portion due to the potential which is rendered by the movable electrical contact rod serving as the electrically charged portion through a bellows mounted on the end plate to another opposed electrically charged portion, i.e., the cup-shaped metallic casing when the movable electrical contact is drawn away from the stationary electrical contact so that the creepage distance of the bell-shaped vacuum power interrupter unit is shorter than that of the conventional vacuum power interrupter unit described above since the creepage distance of the conventional type corresponds to the distance between each end metal plate, thereby the dielectric strength between the movable electrical contact rod and an open end of the bell-shaped metallic casing via the circular end plate not becoming larger and the opening or closing of a larger current being made difficult.